Two typical types of evaporation devices are the falling film type and the rising film type. In the falling film type, the liquid is forced downwardly along essentially vertical heat transfer walls with steam or vapors being generated between liquid films. On the other hand, in the rising film type, the liquid is forced upwardly along essentially vertical heat transfer walls with steam or vapors being generated between liquid films. In the falling film type as well as the rising film type of evaporation devices, the vertical heat transfer walls may be vertically extending plates or tubes with a hot medium, such as steam, circulating between the heat transfer walls.
Examples of falling film type evaporation devices utilizing tubes are disclosed in U.S. patents 4,076,576 to Marttala and 4,641,706 to Haynie, which are both incorporated herein by reference. An example of an evaporation device utilizing vertically extending heat exchange plates is disclosed in U.S. Pat. No. 4,586,565 to Hallstrom et al, which is incorporated herein by reference.
A major problem occurring in most evaporation assemblies is the collection of solid deposits on the heat exchange surfaces. In particular, when the liquid in the tubes or evaporation area begins to evaporate and the solubility coefficient is surpassed, crystals or gums start to precipitate. Accordingly, the greater the concentration of the liquid, the.greater is the viscosity of the liquid as well. Accordingly, the concentrated liquid tends to move slower, and consequently either the precipitate salt crystals or the gums from the liquid will begin to adhere or stick to the walls of the heat exchange surfaces. Once the crystals or gums begin to adhere to the hot wall of the heat exchanger, additional crystals or gums will begin to adhere at an exponential rate.
These solid deposits or scalings decrease the overall efficiency of the heat exchange by reducing the overall heat exchange coefficient, i.e., the amount of heat exchange per unit of time over a given surface area Moreover, the solid deposits can, over a period of time, build up on the heat exchange surfaces to a point requiring stopping operation and cleaning the heat exchange surfaces. Accordingly, this is a very serious problem which designers or manufacturers have made many efforts to overcome.